90 THE ORIGIN OF PARASITES 



Sderostoiiium ecjuinum, in a provisional situation. To the 

 former class, among others, belong certain Strongylidge. 

 (2.) The embryos iind, in pursuance of an active or passive migration 

 — without ever having led a free life — an intermediate host, in 

 the organs of which they develop into a larval form, which then 

 ends its life-history under various circumstances. ^ 

 (a.) The larva migrates, and becomes, after complete metamorphosis, 

 a free-living animal (Oestridas among other flies, Ichneu- 

 monidae, Mermithidse, Gordiacese.) 

 (6.) The larva arrives at sexual maturity in its intermediate host 

 without further metamorphosis, as in the above-mentioned 

 ArcMgetes and Aspidogaster (p. 69). 

 (e.) The larva remains in the intermediate host untU it migrates into 

 its definitive host, mostly in a passive way (with food).' 

 The developmental history in such cases is extended over 

 two different hosts. This is the form of life-history which 

 we discover in the majority of intestinal worms, and may be 

 considered as really typical among Entozoa (Cestodes, with 

 the exception of ArcMgetes, Acanthoc'ephala, Distomidee, and 

 Pentastomidse). In individual cases there are here certain 

 modifications, thiis — 

 (a.) The number of intermediate hosts increases, whUst the larva 

 either migrates of itself, and seeks a new host (certain 

 Cestodes), or produces asexually a new generation, which 

 then enters upon a similar change of host (Distomidse). 

 {j3.) The intermediate and definitive hosts become one and the 

 same when the embryos do not forsake the latter, but 

 simply wander into its peripheral organs, and there develop 

 into larvEe {Trichina). 

 (3.) The embryos pass at once, whilst they are yet enclosed by the egg- 

 shell, in a passive manner into the intestine of their definitive 

 host, and here complete their further development. To this 

 class belong numerous Nematodes, especially Triclioceplialus 

 and Oxyuris. 



The order in wliicli we have drawn up the various modifications of 

 ]3arasitic life aff'ords us at tlie same time a picture of the gradual in- 

 crease in development of ^v'hich this life is capable. The first 



' The form of entoparasitio life -whicli is here shortly characterised is that which was 

 earliest known to us, and when the first edition of this work appeared, it was the only one 

 known. The existence of the other forms (1 a. and h., and 3) has only been proved later 

 through my researches, especially with regard to Nematodes. (See Vol. II. ) To these 

 Helminths, which develop according to the newly discovered laws, belong the most impor- 

 tant human parasites ; and yet KUchenmeister (" Parasiten," 2d Ed., Preface) maintains 

 that to the knowledge of parasites " nothing new or of practical importance can be added" 

 beyond what he had asserted ! 



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